Thermally bonded nonwoven fabrics comprising a thermally fused conjugate fiber, containing a low-melting peak component that is exposed at least partially on the surface of the fiber and a high-melting point component that has a melting point higher than that of the low-melting point component, are used in various applications, such as nonwoven fabrics used in hygienic materials, packaging materials, wet tissue, filters, wipers, or the like, nonwoven fabrics used in hard stuffing, chairs, or the like, or molded bodies. In particular, as a urethane foam substitute, there is a growing demand for high elasticity and high bulk recovery property of a nonwoven fabric, that is, a demand for a fiber having high bulk recovery property in the thickness direction. There is a strong demand for a urethane foam substitute because urethane foam is problematic in that, for example, the handling of chemicals used during production is difficult, chlorofluorocarbons are discharged, and disposal after use is difficult. Furthermore, an obtained urethane foam is problematic in that, for example, the feeling when initially compressed is hard, the air permeability is so poor that stuffiness easily occurs, the sound absorbency is insufficient, or the color easily is changed to yellow. Accordingly, various investigations have been conducted on a nonwoven fabric having high elasticity and high bulk recovery property.
Patent Documents 1 and 2 below disclose a conjugate fiber, comprising: a polyester component having a melting point of 200° C. or higher; and a polyether-ester block copolymer component, that is, a so-called elastomer component, having a melting point of 180° C. or lower. Since the sheath component comprises an elastomer component, the degree of freedom in bonding points and the durability when the conjugate fiber is deformed by compression are improved, and, thus, the bulk recovery property is excellent.
Patent Document 3 below discloses an actualized crimping conjugate fiber, comprising: a first component that contains a polytrimethylene terephthalate (PTT)-based polymer; and a second component that contains a polyolefin-based polymer, in particular, a polyethylene, wherein crimps are actualized by shifting the centroid position of the first component from that of the fiber in the cross section of the fiber. This actualized crimping conjugate fiber comprises a polymer having large bending elasticity and small bending hardness as the first component, the cross section of the fiber is eccentric, and the crimps are wavy, and, thus, it is possible to obtain a nonwoven fabric that has high bulk recovery property, is flexible, and has a large initial bulk.
Patent Document 4 below discloses a latently crimpable conjugate fiber and a nonwoven fabric, wherein a core component comprises polyethylene terephthalate (PET), a blend of PET and polybutylene terephthalate (PBT), or a blend polymer of PET and PTT, and a sheath component comprises a linear low-density polyethylene (LLDPE) resin polymerized using a metallocene catalyst.    [Patent Document 1] JP H4-240219A    [Patent Document 2] JP H5-247724A    [Patent Document 3] JP 2003-3334A    [Patent Document 4] JP 2006-233381A
Patent Documents 1 and 2 above try to provide a nonwoven fabric having excellent bulk recovery property by using a polyesterether elastomer in the sheath component, the polyesterether elastomer being a polymer that has rubber elasticity and provides a large degree of freedom in deformation at bonding points. However, since this polyesterether elastomer is a copolymer of a hard polyester and a soft ether, and comprises a soft component having low thermal resistance, this polymer easily is softened by heat, and so-called sag occurs in which the bulk of a nonwoven fabric is reduced during heating. As a result, a conjugate fiber in which the sheath component comprises such a polyesterether elastomer is problematic in that the initial bulk when formed into a nonwoven fabric is small, the thus obtained nonwoven fabric always has a high density, and, thus, their applications are limited. Furthermore, a nonwoven fabric that has been compressed with the application of heat, or that repeatedly was compressed is problematic in that, for example, the fiber-bonding points and the fiber itself are broken or bent, or the fiber strength is lowered, that is, the hardness of this nonwoven fabric becomes significantly lower than that of the original nonwoven fabric.
Patent Documents 3 above and 4 try to provide a nonwoven fabric having excellent bulk recovery property by using a specific polymer in the core, making the specific cross section of the fiber specific, and providing a specific crimping state. However, although the initial thickness (initial bulk) of the nonwoven fabric is large, the bulk recovery property, in particular, the initial bulk recovery property immediately after removal of a load is not sufficient, and, thus, there is a problem in that the applications are limited.
That is to say, in conventional examples, a fiber for a nonwoven fabric having a large initial bulk (having a low density) and excellent bulk recovery property has not been obtained.